There are presently many different types of plate and fixture systems for securing two or more bones or bone fragments in relative position so that the bones may fuse or heal, or so that tissue adjacent the bones may heal without disruption from the movement of the secured bones. As used herein, the term bone may refer to a bone, or a bone fragment or portion, and the term may refer to a portion of a bone that is covered with another material, such as the endplates covering the top and bottom surface of a vertebra. These systems have been used to secure spinal vertebrae and, more specifically, cervical vertebrae.
Bone plate systems are typically used to assist or direct spinal fusion or vertebral healing procedures. These procedures promote earlier post-operative patient mobility, decrease a need for post-operative collars, decrease the incidence of graft dislodgement if a graft is used, and improve success in correcting spinal deformities.
Furthermore, these systems have been found to assist in controlling and/or exerting a loading force applied to the surgical site. As used herein, the term fusion refers to the joining of materials, such as bone or graft material, and the fusion site is the entire region in which fusion may be desired. By applying a compressive load, it has been found that bone heals more optimally and with greater integrity, a principle known as Wolf's law.
A shortcoming with bone plates is the backing out or loosening of the screws. If the screws loosen, the bones are not properly secure and may move relative to each other. This may compromise the ability to achieve optimal bone fusion and bone alignment, or it may lead to loss of graft material, and damage or loss of bone. Furthermore, when the plate is a dynamic or dynamized plate, such that at least some screws may move relative to the plate, these issues may be further compounded or exacerbated by a screw backing out.
In order to increase the amount of loading or compressive force, a number of plate designs have been devised. For instance, compression slots have been formed in a plate whereby a screw receiving bore is in the form of a slot with tapered walls, and a screw with a tapered shank is driven against the tapered wall such that a force between the shank and the slot is directed transverse to the shank. Accordingly, that force compresses the screw and the bone to which the screw is connected towards another bone connected to the plate. Another manner for permitting compressive force between joined bones is to utilize a dynamic plate having at least one elongated screw aperture that allows settling of the vertebrae by gravity by allowing at least one secured bone to move slightly relative to the plate. However, heretofore known arrangements of fixed and dynamized apertures in such plates provide less than optimal capacity for controlling the movement and/or compression between more than two levels of secured vertebrae.
One example of a known dynamic bone plate is disclosed in U.S. Pat. No. 6,755,833, to Paul et al. The '833 patent describes a bone plate assembly including a bone support plate having a plurality of paired apertures for receiving screws securing the plate to vertebrae and being curved about both a longitudinal and a lateral axis of the plate. For each vertical row of apertures, a single, long flexible band is secured by retainers within a recessed portion or channel that extends longitudinally for substantially the entire extent of the top surface of the plate. The plate is depicted having increasingly elongated slots extending from a first end, and each slot permits translation relative to adjacent vertebrae and relative to the first end. This lack of constraint between a screw and its respective slot allows a moment arm to extend from each slot to the hole at the first end. Accordingly, forces through the curved plate are magnified. As the channel reduces the effective plate thickness, the curved plate is more prone to deforming under these compressive forces and moment arms. The band itself must be secured with the plate by the retainers, which may result in a rough upper or top surface. Most cervical plates are secured to the anterior of the spine and may be felt by a patient along the esophagus. A rough surface, as well as significant plate thickness, can be uncomfortable or problematic for a patient during swallowing.
It should also be noted that it is common for the curve of a plate to be adjusted for a particular patient's anatomy, such as via bending of the plate by a plate bending tool. The ends of the bands of the '833 patent are secured within end apertures so that, if the curve of the plate is decreased, the band ends may protrude from the plate. Alternatively, if the curve of the plate is increased, the band ends may pull out of the end apertures, and the bands or retainers may become loose from each other or the plate. For an increased curve, the bands may be stretched which reduces their ability to shift as desired to permit screw securement and to return to their natural position and which may impair their general integrity. Furthermore, an increase of the curvature of the plate draws the bands closer to and into the holes, reducing the ability to properly seat the screw underneath the band.
Another example of a known dynamic bone plate is shown in U.S. Pat. No. 6,533,786 to Needham, et al. The '786 patent discloses a plate with a first pair of holes at a first end of the plate and a series of paired holes or slots extending from the first pair allowing for each vertebrae secured by the plate to translate toward the first plate end. To allow for subsidence between each vertebrae, or between each pair of holes or slots, the slots are increasingly large as they are arrayed away from the first hole pair. More specifically, the plate is fixedly secured to a first vertebra by the first pair of holes, and an adjacent pair of holes or slots is secured with a second, adjacent vertebra which is then permitted to translate toward the first vertebra. A third pair of slots is secured with a third vertebra, adjacent to the second vertebra, and is permitted to translate toward the second vertebra.
However, the third vertebra must translate at least as much as the second vertebra, else the second and third vertebrae will separate due to the translation of the second vertebra toward the first vertebra. Thus, the third pair of slots must provide for the translation expected or permitted by the second pair of slots, as well as the expected translation by the third vertebra relative to the second vertebra. Thus, according to this design, each pair of slots must be increasingly longer to permitted the cumulative amount of translation between the vertebra intermediate such pair of slots and the first pair of holes or slots. One problem with such a bone plate configuration is that forces on the plate and screws, such as moment arms from each level of screws to the lowest level, are greater. Another problem is that, though the screws may translate in the slots, this translation is not precisely controlled or predictable, and the greater translation that is required results in a greater ability for graft material to explant due to this lack of control, or be crushed by bearing a greater than desired load.
Accordingly, there is a need for improved bone plates, bone plate systems for retarding screw back-out, and improved tools and methods for utilizing bone plate systems.